In the conventional fracturing of wells, producing formations, new wells or low producing wells that have been taken out of production, a formation can be fractured to attempt to achieve higher production rates. Proppant and fracturing fluid are mixed in a blender and then pumped into a well that penetrates an oil or gas bearing formation. High pressure is applied to the well, the formation fractures and proppant carried by the fracturing fluid flows into the fractures. The proppant in the fractures holds the fractures open after pressure is relaxed and production is resumed. Various fluids have been disclosed for use as the fracturing fluid, including various mixtures of hydrocarbons, nitrogen and carbon dioxide.
Care must be taken over the choice of fracturing fluid. The fracturing fluid must have a sufficient viscosity to carry the proppant into the fractures, should minimize formation damage and must be safe to use. A fracturing fluid that remains in the formation after fracturing is not desirable since it may block pores and reduce well production. For this reason, carbon dioxide has been used as a fracturing fluid because, when the fracturing pressure is reduced, the carbon dioxide gasifies and is easily removed from the well.
Lower order alkanes such as propane have also been proposed as fracturing fluids. Thus, U.S. Pat. No. 3,368,627 describes a fracturing method that uses a combination of a liquefied C2-C6 hydrocarbon and carbon dioxide mix as the fracturing fluid. As a lower order alkane, propane and butane are inherently non-damaging to formations. However, this patent does not describe how to achieve propane or butane injection safely, or how to inject proppant into the propane or butane frac fluid. U.S. Pat. No. 5,899,272 also describes propane as a fracturing fluid, but the injection system described in that patent has not been commercialized. Thus, while propane and butane are desirable fluids for fracturing due to their volatility, low weight and easy recovery, those very properties tend to make propane and butane hazardous.